In this Thought Leader interview, Prof Rohit Karnik from MIT tells Will Soutter about his work on graphene membranes. Prof. Karnik's team recently discovered that graphene produced by chemical vapour deposition is not a perfect, flawless surface, but contains intrinsic nanopores, which may allow graphene to be used for selective membranes with very high flow rates.

WS: Can you give us a brief introduction to your work on graphene membranes?

RK: We are developing functional membranes that use graphene as the selective material. In our recent work, which has been published in ACS Nano, we fabricated graphene membranes where a single 25 mm2 layer of graphene grown by chemical vapour deposition (CVD) is placed on a porous polycarbonate membrane support. We found that the CVD graphene had intrinsic holes in the 1-15 nm size range, which preferentially allowed transport of smaller molecules whilst blocking larger ones.

WS: What first made you decide to work with graphene?

RK: When we started thinking about membranes for water purification more than three years ago, graphene was attractive due to its remarkable mechanical strength and atomic-scale thickness. Research had shown that pristine graphene is impermeable even to helium, but it was also known that graphene can contain defects.

The very low thickness of the membrane is expected to provide high flow rates, while the ability to sustain pore defects meant that graphene had the potential to selectively allow ions or molecules to pass through.

Figure 1. The graphene membrane used by Prof. Karnik and his team. The membrane is 5mm square, on a polycarbonate substrate. Image credit: MIT

These pores can potentially have different sizes and functional groups, which makes graphene quite distinct from existing membranes. We expected it to exhibit interesting transport properties that cannot be easily achieved using other materials. Secondly, the fabrication of graphene membranes is simpler than membranes made from other nanomaterials.

These aspects make graphene a very distinct and promising platform for a new class of membranes, and we therefore started working towards practical realization of graphene membranes.

After we started work in this area, other groups showed the potential of graphene for gas separations and desalination using computer simulations, and a group just recently observed selective gas transport through microscopic graphene membranes. These developments bode very well for the future of graphene membranes.

WS: Why has no one been able to discover and characterize these defects in graphene films before?

RK: Graphene research is dominated by applications in electronics and photonics. In this case, the larger pores that we observed are less important than other features such as grain boundaries, edges, point defects (that are not quite pores), and interfaces. Plus, CVD graphene, which can be synthesized over large areas, performs poorly for electronic applications and is less favoured over more pristine forms of graphene

However, CVD graphene is likely to be used in membranes due to its scalability and ease of manufacture. The larger pores that allow molecules through were therefore only evident when actual transport measurements were made on membranes fabricated from CVD graphene.

Secondly, imaging graphene is notoriously difficult due to its transparency to electrons, propensity to get damaged by voltages used in most transmission electron microscopes, and tendency to attract contaminants when an electron beam shines on it during imaging.

Achieving atomic resolution on graphene samples requires specialized microscopes that are very rare, and not easily accessible to researchers. We were fortunate to collaborate with Dr. Idrobo of Oak Ridge National Labs, which enabled us to visualize these pores with atomic resolution.

WS: Can you outline some of potential applications of this discovery?

RK: I see our work as a first step in the development of practical graphene membranes. The membrane that we have made perhaps has limited utility in its present form, but the work demonstrates the feasibility of creating large-area graphene membranes where selectivity is imparted by nanometer-scale pores in an atomically thick layer of graphene.

Figure 2. A high-res STEM image of the graphene pore from Oak Ridge National Laboratories. The image is about 32nm on a side, making the pore about 10nm across. Image credit: Juan-Carlos Idrobo, ORNL.

The field is wide open from here, especially as more control is achieved over the pores in graphene. Potential applications include filtration of biological or other samples with reduced processing times, high flow rate water filters to remove pathogens or contaminants, water desalination, gas separations, and others.

WS: Larger-scale commercial application of graphene membranes will no doubt be some way off - are there any smaller scale applications or research avenues which will be opened up for exploration?

RK: I think there may be some accessible practical applications on a small-scale - for example, molecular weight cut-off membranes for biological applications. In this case, graphene membranes may significantly cut down on processing time due to the high flow rate. Biological samples are expensive, and the membranes are disposed of after first use to prevent contamination, so the advantages available here are potentially huge.

Another possibility is portable water filters. The bar for these kinds of applications is much lower than more entrenched applications like large-scale reverse osmosis desalination membranes. Graphene membranes are likely to penetrate these smaller markets first. But then again, who knows what new innovations and applications may come up?

WS: How can the properties of pores in graphene be controlled to suit these widely varying scenarios?

RK: These are several possibilities to control the pores in graphene. A group from the University of Colorado recently showed that a type of oxidative etching imparts gas selectivity to microscopic areas of graphene. Others have shown that ion or electron bombardment can create and grow pores, but again on microscopic areas.

Can similar methods be applied to these larger area membranes? Yes, if we employ some clever approaches to minimize the flow through the intrinsic defects. What we found is that the intrinsic defects account for less than 1% of the graphene area. That leaves the remaining 99% to be engineered with pores. These pores may further be functionalized with different chemical groups and materials to tune their selectivity.

The key to achieving controlled transport properties through tailor-made pores will lie in the development of membrane architectures that eliminate or minimize the “leaks” by appropriate choice of support materials, or by other means.

WS: What will next steps in your own research be?

RK: We are currently working on methods to controllably generate pores and to demonstrate actual flow-through filtration of small molecules or salts through graphene membranes (in the present work we focused on diffusion through the membrane).